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PROCEEDINGS OF THE NATIONAL MUSEUM 



resemblances to be with the southern Pseudemys rubriventris. From 

 this species, however, the fossil is at once distinguished by its larger 

 size, broader pterygoids, less pronounced median alveolar ridge with 

 finer denticulations, wider interorbital space, and squarely truncate 

 nose. The lower jaws are similar in having the lower surface flattened 

 and alveolar surfaces broad all around. The fossil mandible differs 

 in the absence of a median longitudinal ridge and also in having the 

 edge of the mandible nonserrated. Although in U.S.N.M, No. 12059 

 the beak is notched medially, there is no evidence of cusps on either 

 side, as in P. rubriventris or P. mobiliensis. 



vrnx 



Figure 4.— Skull of Pseudemys idahoensis, U.S.N.M. No. 12059. Type. A, Superior view; B, inferior 

 view. 60, Basioecipital; bs, basisphenoid; exoc, ex occipital ;/r, frontal; j«, jugal; ttix, maxillary; 0, orbit; 

 oc, occipital condyle; p, parietal; pal, palatine; pf, postfrontal; pnu, premaxillary; prf, prefrontal; 

 pr. ot, pro-otic; pt, pterygoid; gj, quadratojugal; qu, quadrate; sg, squamosal; sup, supraoccipital; r, 

 vomer. Natural size 



The hyoid apparatus is well developed. It consists of a more or 

 less oblong flattened basilingual plate, which probably represents the 

 fused ventral ends of the hyoid and branchial arches. Anteriorly it 

 is drawn out to a point behind which there is an ovate opening 

 through the bone on the median line. The second branchial arch is 

 much the larger of the two structures. (See fig. 5.) The posterior or 

 second branchial arch consists of two shortened bars that articulate 

 with the bilobed end of the basilingual plate. So far as I have been 

 able to ascertain this is the first extinct turtle to have the complete 

 hyoid apparatus preserved. 



The pectoral and pelvic girdles remain articulated within the 

 carapace, and for that reason they are not available for critical com- 



